Part of the reason may be because the President has over the years voiced or demonstrated disdain for many of the less fortunate, i.e., women, the disabled, racial minorities, religious minorities, the transgendered, etc. While he may have voiced at one time for gay rights, sometimes people can't (or won't) discern among the various groups and feel it fair to discriminate and/or attack everyone they wish. The President seldom, if ever, comes out and defends any group.